Various kinds of implantable medical leads for providing stimulation to selected body tissue have become available. For example, an implantable cardiac lead delivers electrical therapy to a patient's heart through one or more electrodes on the distal end of the lead. The electrodes are connected via electrical conductors to a connector assembly on the proximal end of the lead. The connector assembly is in turn coupled to an implantable medical device (IMD) such as a pacemaker or an implantable cardioverter/defibrillator (ICD) or to an IMD combining both pacemaker and ICD functions.
Presently available transvenous defibrillation leads typically employ shocking electrodes composed of helically wound coils. These electrodes may include at least one coil that may be made of single or multifilar wire, or of multiple, braided coils each of which may be made of single or multifilar wire.
It is important that the electrical resistance of the defibrillating electrode be minimized so as to minimize I2R losses and maximize the energy delivered to the surrounding tissue so as to preserve battery life. Losses of energy within the coil material are manifested by heat generated in the shocking coil that reduces the efficacy of the cardioverting and/or defibrillating shock. Typical helical coil shocking electrodes are made of solid platinum/iridium alloy wire or platinum clad MP35N alloy. Although having satisfactory corrosion and fatigue resistance, these materials have relatively high electrical resistances, for example, about 3 and about 7 ohms/ft., respectively. Electrode coils made of drawn filled tube (DFT) or drawn brazed strand (DBS) MP35N filled with silver have satisfactory electrical properties but are potentially toxic due to the silver and exhibit low fatigue life.
In addition to electrical considerations, a shocking electrode must have sufficient mechanical strength and fatigue resistance to withstand the repetitive motion of the beating heart over the device's life typically measured in years. Furthermore, the electrode material must resist corrosion and be chemically inactive so as to preclude toxic reactions. Materials such as silver, copper and nickel can be toxic and are susceptible to corrosion.
Accordingly, a shocking electrode should comprise a carefully selected combination of electrical, mechanical and chemical attributes for optimum shocking efficiency and long life.